El Morro’s twisty ternary bastions

While vacationing in San Juan, Puerto Rico this week, I came across a tricky triangular staircase. It’s in a bastion of the Spanish fort El Morro. Military engineers, evidently well-versed in geometry by the late 16th century when these fortifications were built, favored polygonal designs because the oblique angles resisted cannon balls. Triangular outworks provided fields of fire along adjoining walls. In the jargon of fortress design these are called “demilunes” if inside the surrounding ditch or moat, and a “ravelin” if outside. I think even the 5th graders who outsmarted their adult contestant in knowledge of a trapezoid would be challenged to identify all the shapes in a fort like El Morro.

“Sometimes things happen in the world that we are not capable of understanding.” “Yeah, like geometry.” 3/23/07 comic strip Baldo

It turns out that the only one to successfully take over El Morro was Sir George Clifford, 3rd Earl of Cumberland. His English predecessor Sir Francis Drake failed to defeat the Spanish in San Juan three years earlier. Perhaps Clifford’s solid education in mathematics helped him formulate a successful strategy. However, although he was fearsome on a horse -- being one of the top jousters of the age, Sir Clifford struggled on foot. While suited up in his legendary armor, he slipped while bridging a moat and nearly drowned. After traversing the pictured triangular stairway (termed “ternary” as explained in my previous blog), I cannot see how anyone could manage this in full armor. Perhaps by then Sir Clifford had achieved victory and stripped down to his Bermuda shorts to sip on one of San Juan’s famous pina coladas.

For more on El Morro and its place in the battle against pirates of the Caribbean, see this blog by scholar Jon Beasley-Murray.